This invention relates to a charge transfer device fabricated on a semiconductor substrate. In particular, this invention relates to a structure of an output section of the charge transfer device.
A charge transfer device comprises a charge-coupled device (CCD), a junction type field-effect transistor, and a reset transistor. The charge-coupled device, the junction type field-effect transistor, and the reset transistor are fabricated on a semiconductor substrate.
The charge-coupled device comprises a transfer channel region isolated by a channel stopping region of a first conductivity type and diffused with a second conductivity type into a well region of the first conductivity type. The well region is formed on a surface of the semiconductor substrate of the second conductivity type. An output gate electrode is arranged on a first insulation film formed on a surface of the transfer channel region. The output gate electrode has its edge aligned with an output end of the transfer channel.
The junction type field-effect transistor comprises a junction gate region formed with the second conductivity type in contact with the transfer channel region at an output end of the charge-coupled device. A source region of the second conductivity type reaches the well region in contact with the junction gate region. In a conventional charge transfer device, a drain region is a part of the channel stopping region. The source region is connected to a load and an output terminal of the charge transfer device. The drain region is grounded. A hole current flows from the source region to the drain region through the well region.
The reset transistor comprises a reset gate electrode adjacent to the junction gate region. The reset gate electrode has its edge aligned with an edge of the junction gate region. The reset gate electrode is arranged on a second insulation film interposed between the junction gate region and the reset gate electrode.
The charge-coupled device transfers electric charges supplied from an input device, such as a CCD image sensor. The charge-coupled device supplies the electric charges to the junction gate region of the junction type field-effect transistor. The junction gate region stores the electric charges as stored electric charges. The stored electric charges modulate the hole current. An electric potential of the output terminal varies according to the stored electric charges. The electric potential is detected as an output signal. The reset transistor resets the junction gate region of the junction type field-effect transistor.
It is difficult in the conventional charge transfer devices to align the edge of the output gate electrode correctly with the output end of the transfer channel region, and to align the edge of the reset gate electrode precisely with the edge of the junction gate region. Therefore, manufacturing errors are likely to occur.
Furthermore, conventional charge transfer devices are defective in that reset noise appears in their output signals. Moreover, the output signal has an output characteristic which is influenced by the storing of electric charges as a result of manufacturing error.